A digital signal is a modulated signal according to a coding of one and zero, and may be a rectangle pulse signal, for example. If the coding is NRZ (Non Return to Zero), the digital signal is generated as a pulse signal that “0” is assigned to L (Low) level and “1” to H (High) level every UI (unite interval). FIG. 1 shows an example. If a propagation path between transmitter and receiver is ideal, the waveform of the rectangle pulse signal would not change after the propagation and then would be an ideal waveform shown with a dotted line. The receiver detects values of the rectangle pulse signal every UI to demodulate the 1 and 0 data.
An actual transmitted signal usually has distortions relative to the waveform of an ideal signal depending on characteristics of the propagation path and signal speed, as shown as solid lines in FIG. 1. That is, the information the digital signal transfers is digital data comprising 1s and 0s but the signal itself is an analog signal. The waveform of the digital signal is displayed as an eye pattern with a waveform display apparatus, such as a digital oscilloscope, that samples and stores an input signal as sample data to measure quality of the signal.
FIG. 2 is a functional block diagram of a waveform display apparatus, such as an oscilloscope. A CPU 10 controls operations of the apparatus according to programs stored in a hard disk drive (HDD) 14. The CPU 10 makes a memory 12 read data as necessary to conduct processes. The HDD 14 stores programs realizing functions necessary for analyzing a signal under test such as an eye pattern display in addition to operating software (OS) that controls basic operations of the apparatus. They are coupled via a bus 16. They may be the same devices as used in a personal computer (PC).
Operation panel 18 and mouse 20 are coupled to the bus 16 via an I/O port 22. A user can set up the waveform display apparatus via them. An external storage device 24 can be coupled via an I/O port 22 and the sample data of the signal under test, generated by the waveform display apparatus, may be copied to the external storage device 24. If the external storage device 24 is decoupled and coupled to a PC (not shown), then the stored sample data can be copied to a HDD of a PC.
A preamplifier 26 properly adjusts the signal under test and provides it to analog to digital converter (ADC) 28 and trigger detector 30. The ADC 28 samples the signal under test at a frequency that is well higher than the frequency of the signal under test to generate the sample data. A fast acquisition memory 32 stores the sample data sequentially. When fast acquisition memory 32 is full, the older data is deleted sequentially. When the trigger detector 30 finds a portion matching its trigger criteria (i.e., a trigger condition in the sample data), it controls acquisition memory 32 to store data surrounding the trigger event (i.e., pre-trigger and post-trigger sample data) until acquisition memory 32 is full and thereafter, stops the acquisition. The acquired sample data may be transferred to the HDD, if necessary. CPU 10 processes the sample data in a bitmap form to generate an eye pattern as shown in FIG. 3. A display apparatus 34 displays the sample data as a waveform along a time axis or as an eye pattern according to user settings.
Recent waveform display instruments often adopt the same OS as a personal computer (PC); such an instrument are the DPO-series of oscilloscopes manufactured by Tektronix, Inc., Beaverton, Oreg., U.S.A. Therefore, an optional program for the waveform display apparatus may be developed with a PC and installed on the waveform display apparatus to provide an additional feature of the waveform display apparatus. Conversely, if the same program for processing the sample data as the waveform display apparatus is installed on a PC, a user can conduct the same processes with the PC by copying the sample data, generated in the waveform display apparatus, to the PC. In this way, a waveform display apparatus and a PC resemble each other in that they each have a processing unit such as a CPU.
FIG. 3 is an eye pattern display example derived from sample data of 8 bit patterns from 000 through 111. In the example, timing of bit patterns are aligned, frequency information derived by counting the number of the sample data of each pixel is converted to a color or brightness. The frequency information is indicated with a histogram using a rainbow of seven colors wherein the frequency is higher as the color is closer to red and lower as it approaches violet. An eye pattern may also be referred to as an eye diagram.
In the eye pattern display, large distortions of waveforms lead to a decrease in the area of the formed eye, a large difference from the ideal form. For example, U.S. Pat. No. 6,806,877 discloses a technology that a mask having a shape according to user settings is provided on an eye pattern to evaluate whether the mask touches the eye pattern or not to measure quality of the digital signal.
An eye pattern image is derived by collecting many bit patterns and is very useful for measuring whether the overall quality of a signal under test is good or bad. However, even if a portion of the eye pattern indicates that the signal under test has a problem, it concurrently makes it difficult to identify which portion of the signal under test is specifically that portion. Therefore, it would be preferable to easily determine which portion of a signal under test is a problem portion, found within the eye pattern, while measuring overall quality of a signal under test with the eye pattern display.